1. Field of the Invention
One aspect of the present invention relates to a method and apparatus for rapidly thawing NOx reductant.
2. Background Art
While diesel and lean-burn gasoline engines are known to provide beneficial fuel economy, these engines also produce oxides of nitrogen (NOx) and particulates during normal operation. Combustion can be optimized to reduce exhaust emissions and obtain beneficial fuel economy. However, these measures may not be sufficient to bring NOx levels within current and proposed regulatory standards that impose limits on the NOx levels in engine emission streams. Selective catalytic reduction (SCR) methods are known for after treatment of the emissions stream to reduce NOx to acceptable regulatory levels.
SCR methods have been employed to remove NOx from diesel exhaust with a liquid reductant, such as aqueous urea. NOx and ammonia are brought into contact with a selective catalyst and catalytically converted into environmentally safe nitrogen and air. The liquid reductant is typically injected directly into the exhaust pipe, where it decomposes by thermal hydrolysis to ammonia and carbon dioxide. The ammonia stored on the surface of the SCR catalyst reacts with NOx compounds in the exhaust gases to reduce NOx compounds to nitrogen.
One disadvantage of liquid reductants is that freezing of the reductant may occur. The freezing temperature varies relative to the composition and concentration of the dissolved reductant. For example, solutions having a urea content of about 33% in water, typically freeze at about 12° F. (−11° C.). Liquid reductant freezing is particularly a problem for the use of diesel vehicles in cold-weather climates when a liquid reductant is employed to help meet emission standards for NOx. Moreover, regulations may require that urea is available in a liquid state, even at cold temperatures at engine start up, or within a relatively short amount of time from engine startup.
In light of the foregoing, what is needed is a method and system that employs liquid reductants for decreasing NOx emissions in cold weather climates without experiencing at least one of the above-identified disadvantages.